With an explosive Internet population growth in recent years, achievement of a high-speed and large-volume information transmission has been rapidly required, and it is considered that optical communication plays an important role also in the future.
A semiconductor laser device is mainly used as a light source for the optical communication. For example, for short-distance use such as a transmission distance up to about 10 km, a direct-modulation method of directly driving a semiconductor laser by an electric signal is used. This direct-modulation method has a feature of low power consumption because a module can be achieved by a simple structure. Also, low cost can be achieved because of a small number of components.
On the other hand, only the direct modulation of the semiconductor laser cannot handle a long-distance optical communication such as a transmission distance over 10 km, and therefore, an electro-absorption (EA) modulator-integrated semiconductor laser in which optical modulators are integrated is used for the long-distance use. Further, for an optical communication such as a distance over 80 km, a Mach-Zehnder (MZ) modulator having small chirping is used.
Basic structures of the optical-communication semiconductor laser are briefly categorized into two types of a ridge-waveguide (RWG) structure and a buried-hetero (BH) structure (hereinafter simply referred to as a buried structure). FIGS. 1A and 1B are cross-sectional views along a direction of mesa stripes of these two types of structures.
In a ridge-waveguide structure illustrated in FIG. 1A, when an upper cladding layer 104 and a contact layer 105 formed on a semiconductor substrate 101 are etched to form a mesa stripe whose width is several run, an excess portion is removed down to right above an active layer 103 by etching. At this time, the active layer 103 is not etched, and therefore, a reactive current component exists, the reactive current component not injected into the active layer 103 to be spread at a current injection.
On the other hand, in a buried structure illustrated in FIG. 1B, when a mesa stripe is formed, not only the upper cladding layer 104 but also the active layer 103, a lower cladding layer 102, and the semiconductor substrate 101 are etched, and a buried layer 106 formed of a semi-insulating semiconductor is grown on both sides of the mesa structure. In this case, since a current can be efficiently injected only to the active layer by the high-insulating buried layer 106, a laser can be operated with a lower threshold current than that of the ridge-waveguide structure in principle.
In a manufacture of a conventional InP-based buried semiconductor laser device, a metal-organic vapor phase epitaxy (MOVPE) method capable of high-quality buried regrowth has been mainly used as a method for growing a semiconductor layer. Also, as a material for the high-insulating buried layer, Fe-doped InP has been used. However, since Fe has a property of crossdiffusion with Zn normally used as a p-type dopant, there are problems such that Zn diffuses from a p-type cladding layer to the buried layer to damage insulation properties, and conversely, Fe diffuses from the buried layer to the cladding layer to decrease conductivity.
Conventionally, the above-described Fe—Zn crossdiffusion is suppressed by decreasing a doping concentration of Fe as little as possible. Therefore, a current blocking effect is insufficient, and a leakage current component not injected into the active layer occurs, and therefore, an expected effect in the conventional buried semiconductor laser cannot be sufficiently obtained.
For such a problem, Dadgar and others have newly reported Ru as the semi-insulating dopant in place of Fe in 8th International Conference on MOVPE (8th International Conference on MOVPE, abstract, PDSP. 7, 1996). Thereafter, it was confirmed also through an experiment that Ru does not cross-diffuse with Zn, and it has been reported that a Ru-doped InP-based buried structure has characteristics superior to that of a conventional Fe-doped InP-based buried structure.
However, the above-described structure using the Ru-doped InP as the material for the semi-insulating buried layer does not have a sufficiently-large band gap with respect to the active layer and the InP cladding layer, and therefore, a leakage current in the buried layer is not sufficiently suppressed. Accordingly, to solve this problem, a technique has been suggested in which a wide-gap layer having a larger band gap than that of the InP cladding layer is provided between the InP cladding layer and the InP semi-insulating buried layer to suppress the leakage current.
FIG. 2 is a cross-sectional view along a direction of mesa stripes of a structure provided with the wide-gap layer. After a normal mesa structure is formed on the lower cladding layer 102, first, both sides of this mesa structure are buried with a wide-gap layer 107, and next, are buried with the buried layer 106.
As a prior art of the structure provided with the wide-gap layer, there are documents such as Japanese Patent Application Laid-Open Publication No. H01-302791 (Patent Document 1) and Japanese Patent Application Laid-Open Publication No. 2002-314196 (Patent Document 2). In Patent Document 1, an undoped InGaP layer having a larger band gap than that of an InP cladding layer or a Fe-doped InGaP layer is provided between an InP semi-insulating buried layer and the InP cladding layer to suppress a leakage current into the buried layer. And, in Patent Document 2, a Ru-doped InAlAs layer is provided as a wide-gap layer to suppress the leakage current.